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Old 28-05-2012, 19:05   #46
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Re: Cooler Engine Intake Air

Originally Posted by rabend View Post
Thanks for your comment. By coincidence, I am also a registered PE. What I mean by objective, is objective data. An exemplar would be: Intake air temperature rises 20 degrees; horsepower and/or efficiency decrease by 2%. I am not sure if the latter is a linear function, so a graph might be helpful. That kind of information may not be available though.

This has been an interesting discussion containing aspects I had not fully considered.

its all going ot vary based on lots of factors including prop pitch & how your boat responds to power increases and how close to the hull speed you start your test. You know of course that HP output and efficiency is a function of rpm. If you post it on a dyno you get a nice bell shaped curve of tons of data - meaning - there is no definite answer you can calculate four your boat. I can change the whole deal on our boat by changing the prop pitch. I watch the rpms, exhaust stream temperature, throttle, and pitch against boat speed and exhaust color. I can get it to about 4 km/gal for our 55,000 disp.

In math terms, all else fixed, use the gas laws to predict the variation in O2 mass for the tempertaure difference. Remember to use absolute temperature. You will note that the percentage difference is not nearly as important as all of the other things you might do (1 degree F colder is nominally 1/460 th higher density. If your engine intake is hot as heck it will make a difference. Outdoor air is probably the coldest but one load of seawater buys a lot of fuel.

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Old 28-05-2012, 19:26   #47
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Originally Posted by Wotname

Old thread I know; pretty sure you are both wrong re water.

There is no way that the engine can break H2O into its components of H & O therefore the O content does not provide additional oxygen. Even if it could be broken into H & O, the H component would then be a fuel which would recombine (ie burn) with the O. The energy released in burning the H & O would be less than the energy required to break it apart in the first place - unless of course you guys are making the first peptual motion machine

What the water (liquid water, not water vapour) does do however is to increase the compression ratio as you are introducing an incompressible liquid into the cylinder. The cooling effect is as you have stated, a good thing.

Water vapour being a gas and thus compressible has the reverse effect, it decreases the amount of O2 available as it takes takes up space during the intake cycle that would be better taken up by air.

Hence high humidity (water vapour) is bad while fog (water as liquid drops) is good.
I stand corrected on water injection. Primary purposes are cooling and increasing density of the air/fuel mass. Nice catchnin keeping me honest.

In regards to performance I know there is a thread somewhere where someone posted some "real" numbers. I can' find it but if I recall the upshot was that at small displacements the HP gains were minimal. Of course cooler is better but on a 50hp engine that is usually run below redline anyway an extra 1/2 hp probably wont be noticed.

Temperature, pressure altitude and humidity all play a factor in air charge density.

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